We are studying some novel effects of UV radiation, a major environmental carcinogen and mutagen, in animal cells. UV light induces two unique and distinct types of inhibition of the synthesis of small nuclear RNA (snRNA) species U1, U2, U3, U4, and U5. (U1, U2 and U4 RNA, and possibly U5 RNA, each have essential functions.) U snRNA synthesis shows an immediate sensitivity to UV light that is about 20 times greater than that expected from the sizes of the U snRNA transcription units. These are the only known genes whose expression exhibits an immediate hypersensitivity to UV radiation. A second, delayed UV light-induced inhibition of U snRNA transcription requires about 1-2 hr of postirradiation cell incubation and UV doses that are about 10-fold lower than those needed for a comparable level of immediate inhibition, and has not been observed with any other RNA species. Since these two inhibitions are unique, there should be some unique and interesting features in the mechanism underlying each one of these two types of inhibition. Our ultimate goal is to determine the mechanisms of these two types of inhibition. Detailed analysis of the mechanisms of these two inhibitions will require cell-free systems that reproduce them. First, the essential elements needed to produce these two types of inhibition have to be defined, and that is the immediate goal of the present proposal. Some models for the mechanisms of these two inhibitions are compatible with the available results. The critical lesion induced by UV light may be on DNA, but away from the U1 gene region. Two alternative models cannot be rules out now. A small UV dose might produce a disproportionate amount of damage to a certain sequence or molecule, or a small amount of UV damage in a given sequence or molecule might result in a disproportionate loss of function. Then, in spite of the apparent discrepancy in target size, it is conceivable that UV light-induced crosslinking of DNA to another molecule, most likely a protein, or direct damage to a protein, might be the critical lesion in any of these two inhibitions. A sequence of experiments is proposed to test these hypotheses for U1 RNA genes, based on transient expression assays, intranuclear injection of U1 RNA genes into Xenopus oocytes, transcription in isolated nuclei, and transcription in cell-free extracts.